a recent ATLAS analysis found \(R_{\gamma \gamma } = 1.00 \pm 0.12 \)  [1]}.
There is in fact the possibility that the new physics contribution interferes negatively with the SM
amplitude, namely \(\mathcal {A_{\gamma \gamma }^{\rm NP}} \approx -2\mathcal {A}_{\gamma \gamma }^{\rm SM} \approx 13.0\) .
This is obtained either for \({\cal Y}\approx 2\) 
(\(1.93 \lesssim {\cal Y}\lesssim 2.03\)  [2\(\sigma \)  range])
or
\({\cal Y}\approx -1\)  (\(-1.03 \lesssim {\cal Y}\lesssim -0.93\)  [2\(\sigma \)  range]),
both yielding
\(\mathcal {A_{\gamma \gamma }^{\rm NP}}({\cal Y}= 2) = \mathcal {A_{\gamma \gamma }^{\rm NP}}({\cal Y}= -1) \approx 13.3\) .
A correlated signal in the \(\gamma Z\)  channel yields \(\mathcal {A}_{\gamma Z}^{\rm NP}\approx -\frac{2}{3}c_W [1 - (3 - 8 {\cal Y}+ 8 {\cal Y}^2) t^2_{W}]\) , leading to a large deviation
\(\mathcal {A}_{\gamma Z}^{\rm NP} ({\cal Y}= 2) = \mathcal {A}_{\gamma Z}^{\rm NP} ({\cal Y}= -1) \approx 2.33\) 
in the region
where the value of \({\cal Y}\)  is compatible with the di-photon channel. The \(\gamma Z\)  decay channel of the Higgs has not been observed yet and HL-LHC is expected to measure \(\kappa _{\gamma Z}\)  within \(10\%\)  precision [2]}.
Thus the model with a single Higgs doublet
predicts a strong departure of \(R_{Z\gamma }\)  from its SM value, although extended Higgs sectors
can help to tame modifications of Higgs signals (see e.g. [3]}).
